Assembly for measuring a magnetic field, using a bridge circuit of spin tunnel elements and a production method for the same

ABSTRACT

The invention relates to an assembly for measuring a magnetic field, comprising at least a first layer assembly ( 8 ) and at least a second layer assembly ( 10 ), whereby the first layer assembly ( 8 ) and the second layer assembly ( 10 ) have a hard magnetic layer ( 16 ), a layer ( 18 ) which acts as a tunnel barrier and lies adjacent to the hard magnetic layer and a soft magnetic layer ( 20 ) which lies adjacent to the layer ( 18 ) which acts as a tunnel barrier. The layer assemblies ( 8, 10 ) are located in a bridge circuit for determining the electric resistance. The invention is characterized in that the second layer assembly ( 10 ) also has an electrically conductive anti-ferromagnetic layer ( 22 ) lying adjacent to the soft magnetic layer ( 20 ), or an electrically conductive artificial anti-ferromagnet lying adjacent to said soft magnetic layer ( 20 ). The invention also relates to a method for producing the inventive assembly.

[0001] The invention relates to an arrangement for measuring a magneticfield with at least one first layer assembly and at least a second layerassembly whereby the first layer assembly and the second layer assemblyhave a hard magnetic layer, a layer which is effective as a tunnelbarrier adjacent the hard magnetic layer and a soft magnetic layeradjacent the layer which is effective as the tunnel barrier, and wherebythe layer assemblies are arranged in a bridge circuit for determiningthe electrical resistance. The invention further relates to a method ofmaking an arrangement for measuring a magnetic field in which the atleast one first layer assembly and at least one second layer assemblyare produced, whereby the first layer assembly and the second layerassembly have a hard magnetic layer, a layer effective as a tunnelbarrier adjacent the hard magnetic layer and a soft magnetic layeradjacent the layer effective as the tunnel barrier.

[0002] An arrangement of the kind described and a process of the kinddescribed are known. In the publication INFO PHYS TECH No. 24, October1999, BMBF, VDI Technology Center Physical Technologies there isdescribed how such arrangements can be used for measuring magneticfields. A preferred field of use is for example the detection and themeasurement of movements which are generally of great significance inautomobile technology and in automation technology. Conventionally forsuch position sensing, Hall sensors are used (generally ABS sensors inautomobiles). A new sensor concept utilizes the so-called travellingmagneto resistance effect (GMR-Effect “Giant Magnetoresistance”). Thiseffect is based upon the phenomenon that electrons because of theirquantized spin states are strongly dispersed differently upon passagethrough magnetic layers each in dependence upon the magnetic orientationof these layers. With parallel magnetization, the dispersion is lessthan with magnetization which is arranged antiparallel. This gives riseto a change in the electrical resistance as a function of the externalmagnetic field in which the layer arrangement finds itself. Thesignificance for a position sensor is that the outer magnetic field canbe influenced by a movable element (generally a rotor or also alinearly-movable element) whereby these influences can be indicated asthe electrical resistance in the layer arrangement.

[0003] Basically the layer construction is so chosen that one magneticlayer is configured as a measuring layer and another magnetic layer as areference layer. With this arrangement, which is based upon the GMReffect, the intervening layer between these magnetic layers is a metallayer. If one replaces the metallic intermediate layer in the layerstructure by a thin electrically-insulating layer, like for exampleAl₂O₃, one can obtain a magnetic tunnel component in which the tunnelcurrent is switched in a manner similar to the current in the metallicGMR element. The advantage of the TMR element resides in a still highersignal level and in an extremely low requirement for an active componentarea.

[0004] There are high requirements for the measurement of the resistanceof the layer arrangement since the resistance is the decisive parameterfor the conclusion as to the magnitude and/or the direction of themagnetic field to be measured. It is known that resistances can bemeasured especially precisely with a Wheatstone bridge circuit. This ismainly based in temperature compensation requirements. Additionally witha Wheatstone bridge there is an output voltage which is symmetrical withrespect to a zero point. In the construction of a Wheatstone bridge, thequality of the measurement is dependent upon the precision of theresistances used. Fixed resistances are determinable with practicallyoptional precision. If one however utilizes a plurality of layerarrangements in the bridge circuit, their electrical resistances must bedetermined with precision or established with precision.

[0005] The invention has as its object, therefore, to provide anarrangement for measuring a magnetic field and a manufacturing processsuch that the electrical characteristics of the elements participatingin the measurement can be known precisely or established with precision.These objects are achieved with the features of claims 1 and 12.

[0006] The invention builds upon the arrangement described at the outsetin that the second layer assembly has, in addition, anelectrically-conducting antiferromagnetic layer adjacent the softmagnetic layer or an electrically-conductive synthetic antiferromagnetadjacent the soft magnetic layer. In this manner, the second layerassembly, which is used as a resistor in the Wheatstone bridge circuit,can be designed like the first layer assembly. By means of theadditional electrically-conductive antiferromagnetic layer, the secondlayer assembly is rendered magnetically insensitive. Such a “pinning”with the second layer assembly has significant advantages by comparisonto a magnetic shielding. The last, which also can be used to reduce themagnitude sensitivity, requires the deposition of thicker high μ layersin the micrometer range. This has significant disadvantages in theproduction of the components. Preferably the bridge circuit is aWheatstone bridge circuit.

[0007] In an embodiment of the invention, the four resistances of theWheatstone bridge circuit are configured as first or second layerassemblies with at least one of the layer assemblies configured with thesecond layer assembly. With such a “full bridge”, a greater stroke ofthe measurement signal can be generated.

[0008] In another embodiment, one resistance of the Wheatstone bridge isconfigured as a first layer assembly, one resistance of the Wheatstonebridge is configured as a second layer assembly and two resistances ofthe Wheatstone bridge are configured as fixed resistors. Since the twofixed resistors do not contribute to the bridge output signal of the“half bridge” the attainable stroke of the bridge is reduced, althoughwith a precision selection of the fixed resistance, there can be abetter reduction of the offset (signal voltage without applied fieldconditioned on resistance fluctuations over the wafer) of the bridge.While this can be realized in conjunction with tunnel contacts notwithout problems, it is conceivable for the hard magnetic layer of thelayer arrangement to be an electrically-nonconductive antiferromagneticlayer. In this manner a reference magnetization can be supplied which isonly limitedly influenced by the controlling outer magnetic field.

[0009] It can be advantageous to pin the hard magnetic layer of thelayer assembly with an electrically-nonconductive antiferromagneticlayer. The hard magnetic layer can be selected in such manner that thebest characteristics with respect to the tunnel contact are obtainablewhile it contains because of the pinning with the antiferromagnet, asufficient counterfield stability and thus can be affected only in amagnetic manner.

[0010] It can be advantageous for the electrically-nonconductiveantiferromagnetic layer to be comprised of NiO_(x). NiO_(x) is suitableas a nonconductor since in tunnel contacts, the current flow runsperpendicular to the substrate.

[0011] In another embodiment, the electrically-nonconductiveantiferromagnetic layer is comprised of a synthetic antiferromagnet.Synthetic antiferromagnets (AAF) have good counterfield stabilityespecially at high operating temperatures.

[0012] It is preferable for the synthetic antiferromagnet to have alayer system of Cu and Co or Ru. By the appropriate choice of the Culayer thickness the antiparallel locations of the Co layers and Rulayers are achieved which can only be removed at very high magneticfields. If one selects the thickness of the Co or Ru layers additionallysuch that the resulting magnetization is small, then the preferredmagnetization direction of the synthetic antiferromagnet can only bechanged with difficulty in an external magnetic field.

[0013] Preferably the electrically conductive antiferromagnetic layer iscomprised of IrMn. In this manner it can act simultaneously to providethe requisite electrical conductivity function and also has theantiferromagnetic serving for pinning action. Preferably the tunnelbarrier is comprised of Al₂O₃. Advantageously, the layer is so made thata thin Al layer is sputtered onto the hard magnetic layer. The sputteredAl layer is then reacted with an oxygen atmosphere of about 10 m bar forabout 30 minutes, whereby a mercury low-pressure lamp is used togenerate UV radiation and has the effect of converting the molecularoxygen into high reactive atomic oxygen and ozone. The oxidation is socarried out that as much as possible all of the deposited Al iscompletely oxidized. A plasma oxidation of the Al is also possible.

[0014] The invention includes a process in which the layer arrangementsare formed parallel to one another in a single procedure and on thesecond layer arrangement additionally, an electrically-conductiveantimagnetic layer is provided adjacent the soft magnetic layer or anelectrically-conductive synthetic antiferromagnet is provided on andadjacent the soft magnetic layer. Through this process it is achievedthat both the first and the second layer arrangements have the sameresistance and electrical characteristics. This has been found to beespecially suitable for use in a Wheatstone bridge circuit.

[0015] Preferably the layer arrangements are produced by deposition ofthe layers. This is an approved method of producing such components.

[0016] It is especially preferably when the additionalelectrically-conductive antiferromagnetic layer is applied inconjunction with the tunnel contact layer system. The fabricationprocess of the layer arrangement can thus be effected withoutinterruption.

[0017] Then the finished layer assemblies, preferably are connected in abridge circuit.

[0018] The invention is based upon the surprising discovery that themeasurement of magnetic fields through the additional application ofelectrically-conductive antiferromagnetic layer on the magnetic layer inconjunction with a bridge circuit gives especially precise results. Thebasis for this, among other things, is that identical layer assemblieswith identical electrical characteristics can be used both as referenceelements and as magnetic field sensitivity elements. This has a specialadvantage also from the point of view of the process since theadditional conductive antiferromagnetic layer of different layerassemblies can be produced in the same fabrication process.

[0019] The invention is described with reference to the accompanyingdrawing for exemplary embodiments.

[0020]FIG. 1 shows a first layer assembly for an arrangement inaccordance with the invention;

[0021]FIG. 2 shows a second layer assembly for an arrangement accordingto the invention;

[0022]FIG. 3 shows the arrangements according to the invention.

[0023] In FIG. 1 a first layer assembly 8 is shown for an arrangement inaccordance with the invention. On a substrate 12, a nonconducting orconducting antiferromagnetic layer 14 is disposed, for example, asynthetic antiferromagnet. This layer 14 can be formed for example byNiO_(x) or IrMn. On this layer is found a hard magnetic layer 16. Thisis followed by a tunnel barrier layer 18, for example of Al₂O₃. On thelayer 18, effective as a tunnel barrier, there is finally a softmagnetic layer 20.

[0024] In FIG. 2, a second layer assembly 10 is provided for use in thearrangement of the invention and the layer sequence is identical exceptthat in the second layer arrangement 10, there is additionally on thesoft magnetic layer 20 an electrically-conductive antiferromagneticlayer 22, for example of IrMn. Even including the soft magnetic layer20, the layer assemblies 8 and 10 of FIGS. 1 and 2 can be fabricated ina common process. This has fabrication advantages; further, the layerassemblies 8, 10 are identical up to the layer 20, especially in termsof their electrical characteristics.

[0025] In FIG. 3 the Wheatstone bridge 24 is shown for measuring themagnetic field dependency resistance. In the present case, it is a “halfbridge”, that means that the resistances R3 and R4 are fixed resistanceswhile the resistances R1 and R2 are layer assemblies. If one selects,for example, for the resistance R1 the layer arrangement 8 according toFIG. 1 and the layer arrangement 10 for the resistance R2 according toFIG. 2, which have been produced in the same fabrication process,precise resistance measurements of those precise magnetic fieldmeasurements can be effected.

[0026] In the present description, in the drawing as well as in theclaims, the features of the invention can be taken individually and alsoin optional combination and are important to the inventor's concept.

1. An arrangement for measuring a magnetic field with at least one firstlayer assembly (8) and at least one second layer assembly (10), wherebythe first layer assembly (8) and the second layer assembly (10) have ahard magnetic layer (16), a layer (18) effective as a tunnel barrier andadjacent the hard magnetic layer, and a layer (18) effective as a tunnelbarrier layer adjacent the soft magnetic layer (20) and whereby thelayer assemblies (8, 10) are arranged in a bridge circuit (24) fordetermining the electrical resistance, characterized in that the secondlayer assembly (10) additionally has an 13 electrically conductiveantiferromagnetic layer (22), adjacent the 14 soft magnetic layer (20)or an antiferromagnet or an electrically-conductive syntheticantiferromagnet is adjacent the soft magnetic 16 layer (20).
 2. Thearrangement according to claim 1, characterized in, that the bridgecircuit (24) is a Wheatstone bridge circuit.
 3. The arrangementaccording to claim 1 or 2, characterized in, that each of the fourresistances of the Wheatstone bridge circuit (24) is configured with afirst layer assembly (8) or second layer assembly (10) whereby at leastone of the layer assemblies is configured as the second layer assembly(10).
 4. The arrangement according to claim 1 or 2, characterized in,that one resistance (R₁) of the Wheatstone bridge is configured as thefirst layer assembly (8), a resistance (R₂) of the Wheatstone bridgecircuit (24) is configured as the second layer assembly (10) and thattwo resistances (R₃ and R₄) of the Wheatstone bridge circuit 24 areconfigured as fixed resistors.
 5. The arrangement according to one ofthe preceding claims, characterized in, that the hard magnetic layer(16) of the layer assemblies (8, 10) is an electrically nonconductiveantiferromagnetic layer (14).
 6. The arrangement according to one ofclaims 1 to 4, characterized in, that the hard magnetic layer (16) ofthe layer assemblies (8, 10) is penned with an electricallynonconductive antiferromagnetic layer (14).
 7. The arrangement accordingto one of the preceding claims, characterized in, that the electricallynonconducting antiferromagnetic layer (14) is comprised of NiO_(x). 8.The arrangement according to one of claims 1 to 6, characterized in,that the electrically nonconducting antiferromagnetic layer (14) iscomprised of a synthetic antiferromagnet.
 9. The arrangement accordingto claim 8, characterized in, that synthetic antiferromagnet has a layersystem of Cu and Co.
 10. The arrangement according to one of thepreceding claims, characterized in, that the electrically conductingantiferromagnetic layer (22) is comprised of IrMn.
 11. The arrangementaccording to one of the preceding claims, characterized in, that thelayer effective as a tunnel barrier (18) is comprised of Al₂O₃.
 12. Amethod of producing an arrangement for measuring a magnetic field inwhich at least one first layer assembly (8) at least one second layerassembly (10) are produced, whereby the first layer assembly and thesecond layer assembly have a first hard magnetic layer (16), a layer(18) effective as a tunnel barrier and adjacent the hard magnetic layer(16) and a soft magnetic layer (20) adjacent the layer (18) effective asthe tunnel barrier, characterized in that the layer assemblies (8, 10)are produced in parallel in a single process and that on the secondlayer assembly (10) an additional electrically conductingantiferromagnetic layer is provided on the soft magnetic layer (20) oran electrically-conducting antiferromagnet is provided on the softmagnetic layer (20).
 13. The method according to claim 12, characterizedin that the layer assemblies (8, 10) are produced by deposition oflayers.
 14. The method according to claim 13, characterized in that theadditional electrically-conducting antiferromagnetic layer (22) isapplied in conjunction with the deposition.
 15. The method according toclaim 13 or 14, characterized in that the applied layer assemblies (8,10) are connected in a bridge circuit (24).
 16. The method according toclaim 12 or 13, characterized in that the layer assemblies (8, 10)before finishing are connected in the bridge circuit (24) and that onthem as necessary the electrically-conducting antiferromagnetic layer(22) is applied to the desired elements of the bridge circuit (24).